This invention relates to an improved system for displaying graphic objects over or in conjunction with a video motion picture such as produced from the video signal generated by a video camera. The term "motion picture" is intended to include live television camera pictures as well as those recorded by video recorders or on film.
In U.S. Pat. No. 4,482,893, entitled Cathode Ray Tube Display With Minimized Distortion From Aliasing, filed by the inventor of this application on Feb. 19, 1982, there is disclosed an improved system for displaying graphical objects in a Cathode Ray Tube Display System employing a television type raster scan. The system improves the display of the graphic objects by minimizing distortion caused by aliasing. In accordance with the invention the displayed graphic objects are produced on a constant background color and brightness and the graphic objects are defined in the terms of the transition of the raster scan lines across the edges of the objects. The width of each transition is varied in accordance with the slope that the edge of the graphic object being displayed makes with the horizontal scan line and the brightness of each color is stepped from the starting brightness to the final brightness across each transition representing an edge. However, when the background is not a solid brightness but is instead a varied, textured scene, as would be produced by a TV camera, the calculations of the brightness increments along the transition becomes quite complex. If the background is a motion picture so that the scene is changing with time, the calculation of intermediate transition shades would exceed the capability of current computer hardware.
In large budget commercials, which are prerecorded frame by frame, graphic objects can be produced with smooth sharp edges over a camera picture by a tedious and expensive process. However the use of such a process is impossible in producing graphic objects over a live television picture and is impractical in low budget commercials. The common solution to the problem of generating graphic objects over a camera picture is to use a pixel-based switcher (or keyer), which makes a graphic/camera picture decision on a pixel by pixel basis. Each pixel is either 100% graphics or 100% camera picture signal. In this manner, an inexpensive mixing of graphics and camera picture is achieved. The disadvantage of this system is that edge of the displayed graphic objects have jagged artifacts. These switching artifacts, which are annoying to the viewer, appear as jagged edges within the graphic object shape. This is why when graphic objects such as alphanumeric characters are superimposed over live video pictures, as is frequently done in the videocasting of sporting events, the alphanumeric characters appear with jagged rather than smooth edges. Even if the computer graphics system were able to produce perfectly smooth graphic objects, the switcher system would nevertheless reintroduce a jaggedness around the periphery where the transition from a video camera signal to the graphic signal occurs. Currently there are some systems which try to minimize the effect by blurring the edges between the graphics and the camera picture, but these systems have the undesirable side effect of making the graphic objects look blurred.